Organic conductive polymers have in recent times become more and more widespread in industry. Areas of application are, for example, the through-plating of circuit boards (EP-A-553671), antistatic finishing of photographic films (EP-A-440957) or as electrode in solid electrolyte capacitors (EP-A-340512). Particular importance in these areas has been achieved by poly-3,4-alkylenedioxythiophenes (EP-A-339340), which are distinguished by high stability and electrical conductivity. The monomeric 3,4-alkylenedioxythiophenes necessary for the preparation can in principle be prepared by processes known from the literature. One synthesis is described, for example, in Gogte et al., Tetrahedron 23 (1967) 2437. Starting from thiodiacetic acid diesters I and oxalic acid diesters II, 3,4-dihydroxythiophene-2,5-dicarboxylic acid esters III are prepared in accordance with formula scheme I in the presence of alkali metal alkoxides (step 1). These esters III are then alkylated using dihaloalkanes to give 3,4-alkylenedioxythiophene-2,5-dicarboxylic acid esters IV (step 2), which are saponified to give the 3,4-alkylenedioxythiophene-2,5-dicarboxylic acids V (step 3) and decarboxylated to give the monomeric 3,4-alkylenedioxythiophenes. The isolation of the individual intermediates which has hitherto been carried out in the prior art is complex and expensive. 
In formula scheme 1:
R1, R2, R3 and R4 are identical or different and are a linear or branched, optionally substituted alkyl radical having 1 to 20 carbon atoms,
A is lithium, sodium or potassium, and
Hal is fluorine, chlorine, bromine or iodine.
Surprisingly, it has now been found that steps 1 and 2 or 2 and 3 or 1 to 3 can be carried out without isolation of the intermediates while obtaining the target products in high purity and yield.
The invention therefore relates to a simple process for the preparation of 3,4-alkylenedioxythiophene-2,5-dicarboxylic acids or their esters by combining individual synthesis steps without isolation of the intermediates.
The process is described in greater detail below. When combining steps 1 and 2, the following procedure is followed:
In step 1, the condensation of the thiodiacetic acid esters with oxalic acid esters is carried out in the presence of alkali metal alkoxides. Suitable metal alkoxides are the alkoxides of lithium, sodium and potassium, preferably sodium or potassium, which are derived from linear or branched aliphatic alcohols. Preferred alcohols are methanol, ethanol, isopropanol, n- and isobutanol and tert-butanol.
The reaction is preferably carried out in solution. Suitable solvents are lower aliphatic alcohols, such as methanol, ethanol, isopropanol, n- and isobutanol and tert-butanol. Preference is given to the alcohol which is also present in the alkoxide component. Thiodiacetic acid esters and oxalic acid esters are usually employed in equimolar amounts. Based on 1 mol of the esters, from 2.0 to 4.0 mol of alkoxide, preferably from 2.0 to 3.0 mol of alkoxide, particularly preferably from 2.0 to 2.5 mol of alkoxide, are used.
The reaction is carried out at from xe2x88x9210xc2x0 C. to 200xc2x0 C., preferably from 0xc2x0 C. to 100xc2x0 C., particularly preferably from 10xc2x0 C. to 70xc2x0 C.
The reaction time is from 10 minutes to 24 hours, preferably from 1 hour to 8 hours.
The alkoxide is preferably initially introduced and the esters added dropwise with stirring, either separately or as a mixture.
After completion of the reaction, any excess of alkoxide is neutralized by addition of acids or acidic salts, such as alkali metal hydrogensulphates.
For carrying out the 2nd step, a higher-boiling solvent, preferably having a boiling point of from 100xc2x0 C. to 300xc2x0 C., is then added. Examples of suitable solvents are linear or cyclic amidic solvents, such as N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, aliphatic sulphoxides or sulphones, such as dimethyl sulphoxide or sulpholane. The solvents can be used alone or as mixtures.
After addition of the higher-boiling solvents, the alcohol is distilled off, if necessary under reduced pressure. Subsequently, in a preferred embodiment, a base is added in an amount of from 0.01 to 0.5 mol, based on 1 mol of the esters employed. Preferred bases are sodium carbonate and potassium carbonate. Particular preference is given to potassium carbonate.
The ring closure to give the 3,4-alkylenedioxythiophenedicarboxylic acid ester is then carried out by reaction of the alkali metal salts of the 3,4-dihydroxydicarboxylic acid esters with alkylating agents. Suitable alkylating agents are dihaloalkanes. Preference is given to dichloro- or dibromoalkanes. Particular preference is given to linear 1,2-dihaloalkanes having 2 to 18 carbon atoms and 1,3-dihaloalkanes having 3 to 18 carbon atoms (halogen is identical or different and is fluorine, chlorine, bromine or iodine). Particular preference is given to 1,2-dichloroethane and 1,2-dichlorohexadecane.
The reaction is carried out at temperatures of from 50 to 200xc2x0 C., preferably at from 100 to 150xc2x0 C., if desired under pressure. The reaction duration is from 1 to 24 hours.
When the reaction is complete, the 3,4-alkylenedioxythiophenedicarboxylic acid ester is isolated by removal of the solvent by distillation, if necessary under reduced pressure, and/or precipitation using water. The crude product can subsequently be dried or saponified directly in the moist state to give the free 3,4-alkylenedioxy-thiophenedicarboxylic acid.
In a particular embodiment of the invention, the 3,4-ethylenedioxythiophene-dicarboxylic acid ester is not isolated, with incorporation of step 3. When the alkylation is complete, the majority of the solvent is distilled off, if necessary under reduced pressure, and the 3,4-ethylenedioxythiophenedicarboxylic acid ester is subsequently saponified directly using bases.
The saponification is preferably carried out using alkali metal hydroxides, which are used as a solution in water and/or as a mixture with water-miscible aliphatic alcohols. Examples of suitable alcohols are methanol, ethanol and isopropanol. The saponification can be carried out at room temperature or at temperatures above room temperature. It has proven successful to carry out the saponification at the reflux temperature of the water or the water/alcohol mixture. When the saponification is complete, the free 3,4-ethylenedioxythiophenedicarboxylic acid is liberated and precipitated by addition of mineral acids. The product is subsequently isolated by suction filtration and dried.
In a further embodiment, steps 2 and 3 are combined. In order to carry out the 2nd step, the separately prepared 3,4-dihydroxythiophene-2,5-dicarboxylic acid ester is initially introduced in the solvents described above. As base, from 1.0 to 1.5 mol, preferably from 1.1 to 1.4 mol, of alkali metal carbonate are added, based on 1 mol of 3,4-dihydroxythiophene-2,5-dicarboxylic acid ester. Preference is given to potassium carbonate.
The further reaction to give the 3,4-alkylenedioxythiophenedicarboxylic acid ester and saponification to give the 3,4-alkylenedioxythiophenedicarboxylic acid are carried out as described above under the combination of steps 1 to 3.
The invention is further described in the following illustrative examples in which all parts and percentages are by weight unless otherwise indicated.